Using engineered sand to clean stormwater: A shore thing?

US researchers have developed a new way to remove contaminants from stormwater, which could help water-strapped communities tap into the underused resource.

While recycling stormwater is not a new concept, it is generally non-potable and used for applications such as toilets and irrigating green spaces.

But in a paper published in Environmental Science & Technology, engineers from the University of California, Berkeley, outlined their findings that sand coated with manganese oxide could help purify stormwater in underground aquifers, creating a safe reservoir of drinking water.

Berkeley graduate student Joseph Charbonnet said the way cities traditionally treat stormwater is “broken”, particularly in California, which experienced one of its worst droughts on record from 2012 to 2016.

“We think of [stormwater] as a pollutant, but we should be thinking about it as a solution. We have developed a technology that can remove contamination before we put it in our drinking water in a passive, low-cost, non-invasive way using naturally occurring minerals,” he said.

Charbonnet’s adviser and co-director of the Berkeley Water Centre David Sedlak said although the sand doesn’t remove all types of contaminants, it could be used in conjunction with other water purification systems.

“Before we built the buildings, roads and parking lots that comprise our cities, rainwater would percolate into the ground and recharge groundwater aquifers. As utilities in water-stressed regions try to figure out how to get urban stormwater back into the ground, the issue of water quality has become a major concern,” he said.

“Our coated sand represents an inexpensive, new approach that can remove many of the contaminants that pose risks to groundwater systems where stormwater is being infiltrated.”

To create the coated sand, plain sand is mixed with two forms of manganese that react to form manganese oxide. This binds to organic chemicals such as herbicides, pesticides and bisphenol-A (BPA) and breaks them down into smaller pieces that are less toxic.

The team tested the sand by filtering simulated stormwater containing a low concentration of BPA through the material. The coated sand initially removed nearly all of the BPA but lost its effectiveness over time.

However, the manganese oxide could be ‘recharged’ by bathing the sand in a solution containing a low concentration of chlorine.

Charbonnet said it would take roughly two days to recharge a 50cm deep layer of sand using the chlorine mixture.

“If you have to come in every year or two and dig up this sand and replace it, that is incredibly labour intensive, so in order to make this useful for community stakeholders it’s really important that this stuff can be regenerated in place,” he said.

The team is now performing field tests of the sand in Sonoma County, California, using stormwater from a local creek.

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